Behaviour of CurrVal - can anyone explain this?

Question

Hi All,
I am expiriencing some "strange" behaviour of CurrVal.
I will try to represent it with simple expample.
Let's have one table and one sequence:
create table TestCurrVal (A number, b number);
create sequence SeqTestCurrVal start with 1;and some sample data in the table
insert into TestCurrVal(A, B) VALUES (1,1);
insert into TestCurrVal(A, B) VALUES (1,2);
insert into TestCurrVal(A, B) VALUES (2,1);What I want to do is to select all rows from TestCurrVal table ordered by column A. If the value in this coulmn is different by previous value I want to get next sequence value and if it is the same - to use last sequence value. So expected result is the following one:
         A          B     A_PREV PROBLEMATIC_VALUE
         1          1                            1
         1          2          1                 1
         2          1          1                 2I've tried with this statement:
select a, b, a_prev,
  case
    when a_prev is null or a_prev <> a then SeqTestCurrVal.NextVal
    else SeqTestCurrVal.CurrVal
  end as problematic_value
from (
  select a,
    lag(a) over (order by a) AS a_Prev,
    b
  from TestCurrVal
  order by a
);but the result is
         A          B     A_PREV PROBLEMATIC_VALUE
         1          1                            1
         1          2          1                 2
         2          1          1                 3So it seems that on every row it update the sequence even if case clause is not performed and regardless NexVal is not reached.
Do you have any idea why this happens? ...or/and if you have any workaroud? (but without loosing sequence numbers) (...the only workaround that I have in mind is joining the statement with distinct values and generated sequences for each of those values...but maybe something simple can be introduced)
And finally some "cleanup" statements:
drop table TestCurrVal;
drop sequence SeqTestCurrVal; Best Regards
Message was edited by:
Michail

Answer 1

This is how SEQUENCE works. When it is in the SELECT statement, it is always incremented. http://download.oracle.com/docs/cd/B28359_01/server.111/b28286/pseudocolumns002.htm#i1006157
"Within a single SQL statement containing a reference to NEXTVAL, Oracle increments the sequence once: <...> For each row returned by the outer query block of a SELECT statement."
A simple example:
SQL> CREATE SEQUENCE Test;
Sequence created.
SQL>
SQL> SELECT
  2     Level,
  3     CASE
  4      WHEN 1 < 3 THEN Test.NextVal
  5      ELSE Test.CurrVal
  6     END
  7     A
  8  FROM
  9     Dual
10  CONNECT BY
11     Level <=4;
     LEVEL          A
         1          1
         2          2
         3          3
         4          4
SQL>
SQL> DROP SEQUENCE Test;
Sequence dropped.Basically, Oracle detected the NEXTVAL and will now increment the record for each record. Note, that NEXTVAL and CURRVAL are equivalent in a single statement, because even NEXTVAL stays the same for each record:
SQL> CREATE SEQUENCE Test;
Sequence created.
SQL>
SQL> SELECT
  2     Level,
  3     Test.NextVal,
  4     Test.NextVal
  5  FROM
  6     Dual
  7  CONNECT BY
  8     Level <=4;
     LEVEL    NEXTVAL    NEXTVAL
         1          1          1
         2          2          2
         3          3          3
         4          4          4
SQL>
SQL> DROP SEQUENCE Test;
Sequence dropped.Therefore, it is probably better to decrement the NEXTVAL by 1 (or whatever the INCREMENT BY value is, which can be queried) and exclude the first record.
SQL> CREATE SEQUENCE Test;
Sequence created.
SQL>
SQL> WITH
  2     TestCurrVal
  3  AS
  4     (
  5      SELECT 1 A, 1 B FROM Dual UNION ALL
  6      SELECT 1 A, 2 B FROM Dual UNION ALL
  7      SELECT 2 A, 1 B FROM Dual
  8     )
  9  SELECT
10     A,
11     B,
12     A_Prev,
13     Test.NextVal
14     - CASE
15        WHEN A_Prev IS NULL AND Test.NextVal > 1 THEN 1
16        ELSE 0
17       END Problematic_Value
18  FROM
19     (
20      SELECT
21             A,
22             Lag(A) OVER (ORDER BY A) a_Prev,
23             B
24      FROM
25             TestCurrVal
26      ORDER BY
27             A
28     );
         A          B     A_PREV PROBLEMATIC_VALUE
         1          1                            1
         1          2          1                 2
         2          1          1                 3
SQL>
SQL>
SQL> DROP SEQUENCE Test;
Sequence dropped.